It is well known that, when ammonia and carbon dioxide are subjected to high temperature in a closed system, high pressures are generated and urea is formed. In general, urea and urea synthesis are described in Encyclopedia of Chemical Technology, 23: 548-575 (1983) including the references cited therein, the complete disclosures of which are incorporated herein by reference.
Urea synthesis has been conducted at pressures of from about 100-350 atmospheres in an autoclave maintained at temperatures of 125-250.degree. C. During the synthesis reaction, the ammonia and carbon dioxide combine exothermically to form ammonium carbamate which is then converted into urea and water. In addition to urea and water, the resulting reaction mixture contains uncombined residues of the starting materials and ammonium carbamate. The carbon dioxide and ammonia, which are introduced to the autoclave under pressure, are in either a liquid or vaporous state, while the water, formed during dehydration of ammonium carbamate to urea, forms an absorbent for the ammonia and carbon dioxide. The dehydration reaction takes place in the liquid phase. The conversion of reactants to urea is only partial because of the equilibrium of the dehydration reaction.
The yield of urea from a high pressure synthesis reactor will be appreciably higher, if in passing through the reaction space in a reactor, the liquid phase is passed through the autoclave by plug-flow sometimes referred to as ("piston flow"). The contact between the gaseous phase and the liquid phase results in the condensation of at least part of the gaseous phase. The condensation heat which is recovered, is used for dehydrating the ammonium carbamate to urea.
It has been found that to accomplish the above, the reaction space in a cylindric reactor is subdivided by a plurality of vertically spaced, horizontally disposed perforated reactor trays as is described in, for instance, U.S. Pat. No. 3,046,307, the complete disclosure of which is incorporated herein by reference. With these reactor trays partitioning the inner volume of the reactor, a plurality of compartments or zones is formed, that are consecutively arranged along the direction of the flow of the reaction mixture. Such reactor trays are preferably arranged horizontally within the reactor and cause a uniform mixing of reaction components in each of the zones formed therebetween. The state of the art perforated reactor trays extend horizontally over the entire cross section of the reactor and each contains a plurality of orifices for the passing of the two-phase gas and liquid flow. Since the liquid and gaseous phases pass through the same orifices, the gas flow relative to the liquid flow passing through these orifices is ill-defined, and results in unpredictable random flow patterns of gas and liquid. As a result `stagnant zones` may well be expected in certain areas in a compartment, resulting in a lower conversion.
Another representative means for subdividing the reactor space in a urea synthesis reactor is to use reactor trays with an annular opening between the perforated reactor trays and the internal wall of the reactor as depicted in FIG. 1. Through this annular opening mainly transportation, e.g. fluid flow, of the liquid phase occurs whereas the transportation, e.g. fluid flows of the gaseous phase occurs through in the central portion perforations of each reactor tray. Although the liquid velocity in this annular opening is relatively low, it is possible that part of the liquid flows along the wall to the top of the reactor without being mixed with the bulk of the liquid. This phenomenon is called `bypassing` and is responsible for a smaller conversion of ammonium carbamate into urea compared with that theoretically possible.
Another phenomenon that is responsible for a reduced conversion is `backmixing`. This happens when a liquid flows from an upper compartment to a lower compartment e.g. via the perforations, as seen, for example, in U.S. Pat. No. 4,098,579, the complete disclosure of which is incorporated herein by reference. In the case of backmixing the plug-flow behavior is not optimal and there is an appreciable negative influence on the theoretically obtainable urea yield.
A further increase in urea yield is said to be obtainable by providing urea synthesis reactors with perforated reactor trays with at least one opening for liquid flow on the edge of the perforated reactor tray. These openings for the liquid flow in two adjacent perforated reactor trays are located opposite the central part of the reactor in order to force the liquid flow to pass the central part of the reactor where the transport of the gaseous phase takes place as is demonstrated in Japanese Patent Publication (Kokai) A-38813 (1970), the complete disclosure of which is incorporated herein by reference. In this way a substantially zigzag flow path of the liquid is created which crosses the substantially vertical flow path of the gaseous phase. In this way bypassing is said to be avoided and the urea yield is said to be improved.